This invention pertains to fastener installation, and more particularly to installation of lockbolts in assemblies having sealant or adhesive between the parts that must be squeezed out before the lockbolt is secured.
Lockbolts have been used for many years, particularly in the aerospace industry, to fasten together assemblies of parts. A lockbolt is a high strength fastener capable of rapid installation, interference fit, low weight and repeatable installation parameters conducive to quality control documentation. Lockbolts are bolts, usually made of a titanium alloy such as 6AL-4V Titanium, having a head at a headed end and annular grooves adjacent the opposite (tail) end. A grooved xe2x80x9cpintailxe2x80x9d at the tail end of the fastener provides a gripping portion by which a lockbolt tool can grip the end of the fastener for applying a pulling force. A lockbolt is shown in U.S. Pat. No. 4,221,152, the disclosure of which is incorporated by reference
Lockbolts are installed with a tool that pulls the lockbolt to the desired tensile preload and has a swage ring for swaging an aluminum collar onto the annular groves in the fastener tail for securing the fastener after it has been pulled tight. A narrow neck between the grooved tail and the pintail provides a fracture region designed to break at a predetermined force, about flush with the swage collar, allowing speedy removal of the tool to the next fastener location.
The lockbolt is a fast and reliable fastening system, but suffers from one disadvantage: after securing the swage collar, the fastener cannot be tightened further, or xe2x80x9cretorquedxe2x80x9d, in the parlance of conventional helically threaded bolts and nuts. The grooves in the tail of a lockbolt are annular, not helical. The benefit provided by the annular grooves is that the collar cannot become loose by rotating under the influence of vibration the way a nut can, but if the collar was swaged prematurely, before proper pretension was established, it must be cut off and the lockbolt replaced to obtain a properly pretensioned fastener.
One circumstance in which the lockbolt swage collar occasionally is secured prematurely is when a viscous fluid sealant or adhesive is used in the interface between the two parts to be fastened together, as when they must produce a fluid-tight seal. One example in which sealant is used is in the wing fuel tanks of a commercial jet airplane for fluid-tight seals to prevent leakage of fuel. When an assembly is fastened together with a viscous fluid such as a sealant or adhesive in the interface, the lockbolt is pulled tight on the assembly, but a finite time is necessary before the viscous fluid can flow out of the interface under the pressure exerted by the pulling force of the fastener. This finite time varies, depending on the flow characteristics of the viscous fluid, the temperature, the spacing between adjacent fasteners, the thickness and stiffness of the part material, the pulling force, etc.
Recognizing that it was necessary to allow time for sealant and adhesive to be squeezed out of the interface, manufacturers of lockbolt equipment in the past have provided an intermediate force which mechanics can apply to the lockbolt, during which the sealant or adhesive could flow out of the interface, before application of the final pulling force ramp-up, leading to swaging the collar and fracture of the pintail. The mechanics were expected to operate the lockbolt installation equipment with a sufficient time delay between application of the intermediate pulling force on the lockbolt and the final ramp-up pulling force on the lockbolt leading to the pintail fracture. That time delay would depend on a number of factors, such as the type of sealant, the temperature, the proximity of the fastener hole to the edge of the part. etc.
An experienced mechanic has a remarkably sensitive ability to discern the correct amount of time to allow the sealant to squeeze out of the interface, and high quality assemblies have been consistently produced using the manual approach. However, the commitment of airplane mechanics to produce quality assemblies sometimes resulted in excessive delay times between the intermediate force and the final force ramp-up. Delay times on the order of a two seconds are usually appropriate, but an unusually quality conscientious mechanic would sometimes allow 3 or 4 seconds for sealant squeeze-out, which is more time than necessary. Management was reluctant to admonish such quality conscious mechanics to increase production speed for fear of sending the wrong message about quality vs. production rate, but at the same time management was concerned about excessive production time per operation.
As a consequence, the sealed assemblies produced using lockbolts have continued to be made with excellent quality but at a slower than optimal production rate. An enormous number of such fasteners is installed in a large assembly, such as an airplane wing, so those additional few seconds per fastener accumulates into a significant production time increase, and therefore cost increase, for that assembly. Thus, there has long been a need for a lockbolt installation system that automatically shifts from intermediate pressure to full pressure in the optimal time delay for sealant/adhesive to squeeze out of the interface, allowing full fastener preload to be maintained for the fastener.
Accordingly, this invention provides an improved process for installing lockbolts in assemblies having an interface between two parts in the assembly, with a viscous fluid substance such as an adhesive or sealant in the interface. The invention provides an improved apparatus for installing lockbolts in such assemblies such that an adjustable time delay is provide for exerting a partial fastener preload to allow for fluid squeeze-out before establishing full fastener preload and swaging of the lockbolt collar. The invention produces assemblies fastened at an economical production rate that have fasteners tensioned at the designated preload without relaxation therefrom by fluid squeeze-out.
These features and benefits are attained in a process and apparatus for installation of a lockbolt to fasten together an assembly of at least two parts having a viscous fluid such as adhesive or sealant in the interface between the two parts. A lockbolt is inserted into aligned holes through the two parts and tension is exerted on the lockbolt with a pulling force at a predetermined intermediate magnitude applied by the apparatus. The tension exerted on the lockbolt is reacted against opposite sides of the assembly to exert a compressive force on the assembly to squeeze the desired amount of the fluid out of the interface. The intermediate pulling force is maintained for a precisely timed dwell period based on an optimal period for that fluid and the ambient temperature. After the dwell period, the pulling force is increased in magnitude while simultaneously swaging a collar into threads on the lockbolt to secure the collar on the lockbolt and secure the lockbolt in a state of tension in the hole in the assembly. The pulling force magnitude continues to increase until the pintail breaks off the lockbolt.